function sendGet(url) {
  var xhr = new XMLHttpRequest()
  xhr.onreadystatechange = function () {
    if (xhr.readyState == 4 && xhr.status == 200) {
      //获取服务器响应
      console.log(xhr)
      return xhr
    }
  }
  //发送异步请求
  xhr.open('GET', url, true)
  //发送请求
  xhr.send()
}
